There are a wide variety of thermal processes used in the treatment of petroleum hydrocarbons, particularly heavy hydrocarbon feedstocks. As is well known, these thermal processes are predominantly used for breaking the covalent bonds of the hydrocarbons in the feedstock to convert the feedstock into products that have boiling points lower than the feedstock. Illustrative thermal processes include visbreaking, catalytic hydroconversion, hydrogen donor diluent cracking, fluid coking and delayed coking.
For example, U.S. Pat. No. 4,298,455 discloses a thermal visbreaking process in which a heavy oil is subjected to thermal treatment in the presence of a chain transfer agent and free radical initiator, the combined effect of which is to inhibit the polymerization of lower molecular weight hydrocarbons produced during the visbreaking treatment.
In U.S. Pat. No. 4,378,288 there is disclosed a method of increasing coker distillate yield in a thermal coking process by adding a small amount of a free radical inhibitor.
U.S. Pat. No. 4,642,175 discloses a method for reducing the coking tendency of heavy hydrocarbon feedstocks in a non-hydrogenative catalytic cracking process by treating the feedstock with a free radicalremoving catalyst so as to reduce the free radical concentration of the feedstock.
French Patent 0269515 discloses the use of oxygenated sulfur or nitrogen compound in combination with hydrogen-donating diluents in visbreaking heavy petroleum fractions.
Notwithstanding any advantages the foregoing processes may have, there is need to be able to operate thermal residual conversion processes at ever lower temperatures in order to increase the conversion of feed to desirable products. Unfortunately, as is known in the art, if the temperature of a thermal conversion process is decreased so as to increase the conversion of feed to more desirable products, generally it is necessary to increase the residence time of the feed in the reactor. Increased residence time, of course, results in lowering of production rate, which is undesirable. Decreasing the temperature of a thermal conversion process can have other undesirable effects. For example, in fluid coking, lower temperature conversion typically results in gross agglomeration of the fluid bed of coke and the bed of coke becomes unstable because of the lower cracking rate of the resid feed. On the other hand, if the temperature of the conversion process is raised, production rate will increase but at the expense of forming less valuable gaseous products, such as products boiling below 100.degree. F. Moreover, higher conversion temperatures generally make coke formation at the heated walls of the reactor likely, which is clearly undesirable. Thus, there remains a need for increasing the rate of thermal conversion processes without forming less desirable products and preferably increasing both the rate of conversion and yield of desired products.